1. Field of the Invention
The present invention relates generally to voltage regulator circuits and more particularly to a voltage regulator circuit which can suppress fluctuations of its output voltage.
2. Description of the Prior Art
Japanese Published Patent Gazette No. 53-18694 (corresponding to Japanese Laid-Open Patent Application Gazette No. 46-3527) or the like discloses a voltage regulator circuit in which a reference voltage is set to be equal to an energy gap voltage (1.205 V) of silicon so as to reduce a temperature coefficient to zero. FIG. 1 illustrates an example of such a conventional voltage regulator circuit. This conventional voltage regulator circuit will be described hereinafter with reference to FIG. 1.
As FIG. 1 shows, there is provided a transistor 11 of which the collector and base are both connected to a base of a transistor 12. The collector of the transistor 12 is connected to a base of a transistor 13. The emitters of both the transistors 11 and 13 are directly grounded and the emitter of the transistor 12 is grounded through a resistor 14. The collector of the transistor 13 is commonly connected to a current source 15 and a base of a buffer transistor 16. The emitter of the transistor 16 and the collectors of the transistors 11 and 12 are connected together through resistors 17 and 18. The current source 15 and the collector of the transistor 16 are both connected to a voltage source terminal 1 (Vcc). An output terminal 2 is led out from the emitter of the transistor 16.
As is well known, between a base-emitter voltage V.sub.BE and a collector current Ic of a transistor, there is established a relationship expressed by the following equation (1) or (2). ##EQU1## where Is is the saturation current, q the electron charge, T the absolute temperature and k the Boltzman's constant.
In the known voltage regulator circuit shown in FIG. 1, I.sub.C1 and I.sub.C2 represent collector currents of the transistors 11 and 12, V.sub.BE1 and V.sub.BE2 represent base-emitter voltages thereof and R.sub.14 represents a resistance value of the resistor 14. Then, the following equation (3) is established. EQU V.sub.BE1 =V.sub.BE2 +I.sub.C2 R.sub.14 ( 3)
Applying the equation (2) to the equation (3) yields the following equations (4a) and (4b) ##EQU2##
If a base-emitter voltage of the transistor 13 is taken as V.sub.BE3 and a resistance value of the resistor 18 is taken as R.sub.18, the reference voltage V.sub.REF developed at the output terminal 2 is expressed as in the following equation (5) by utilizing the equation (4b). ##EQU3## From the foregoing equation (2), it is thus apparent that V.sub.T (thermal voltage) has a positive temperature coefficient of about 1/300. Meanwhile, the base-emitter voltage V.sub.BE3 of the transistor 13 is fluctuated in the negative direction with a ratio of about -2 mV/.degree.C. In the voltage regulator circuit shown in FIG. 1, by properly selecting the resistance value of the resistor 18, it is possible to balance the base-emitter voltage of negative temperature coefficient of the transistor 13 and the voltage of positive temperature coefficient produced across the resistor 18 due to the collector current of the transistor 12. Thus, as described above, it is possible to obtain the reference voltage V.sub.REF of zero temperature coefficient equal to the energy-gap voltage of the silicon. If a voltage across the resistor 18 at that time is taken as K.sub.0. V.sub.T, V.sub.T is nearly equal to 26 mV (V.sub.t .apprxeq.26 mV) so that K.sub.0 becomes substantially equal to 23 (K.sub.0 .apprxeq.23).
A voltage regulator circuit mounted on the integrated circuit (IC) is required to have small fluctuation of the output voltage, in addition to the excellent temperature characteristic.
As will be clear from the equation (5), the output voltage from the conventional voltage regulator circuit shown in FIG. 1 depends on the base-emitter voltage V.sub.BE of the transistor 13. This base-emitter voltage V.sub.BE is dependent on the saturation current I.sub.S of the transistor 13 as will be apparent from the equation (2).
In the manufacturing process of the integrated circuit, however, if impurity concentration in the base of a transistor is fluctuated upward or downward, the saturation current Is is decreased or increased dependent on this fluctuation, while on the contrary the base-emitter voltage V.sub.BE is increased or decreased.
As earlier noted, when the output constant voltage from the voltage regulator circuit is set to be V.sub.REF =1.205 V, the fluctuation of the base-emitter voltage V.sub.BE in the general manufacturing process reaches, for example, about .+-.40 mV, i.e., about .+-.3.3%. For this reason, in order to control the output voltage to fall in a predetermined range, the administration of the manufacturing process must be made more strict and the trimming of the resistor or the like must be carried out.